1. Field of the Invention
The present invention relates to a longitudinally-coupled-type surface-acoustic-wave (SAW) resonator filter in which reflectors are provided at both sides of a region where interdigital electrodes are located. More particularly, the invention relates to the above-described type of longitudinally-coupled-type SAW resonator filter having an improved electrode structure in which unwanted spurious responses are suppressed.
2. Description of the Related Art
SAW filters are being used as band-pass filters for various types of communication devices. It is critically important that band-pass filters ensure required pass bandwidths with high selectivity. Accordingly, it is also required that a SAW filter used in a high-frequency stage reliably provide a required pass bandwidth with high selectivity. It is particularly important that RF-stage SAW filters used in cordless telephones having CT-1, CT-1.sup.+, and CT-2 standards obtain attenuations in ranges of .+-.20 MHz and .+-.40 MHz away from the center frequency, though these ranges vary depending on the intermediate frequency (IF) filters used in the telephones. Thus, the RF-stage SAW filters are required to have even higher selectivity characteristics.
On the other hand, resonator-type SAW filters are advantageous compared to other types of SAW filters in terms of decreased insertion losses and increased out-of-band attenuations and also provide a more compact filter. For example, certain types of longitudinally-coupled-type SAW resonator filters are connected in a plurality of stages to sufficiently satisfy the foregoing requirements. In this type of SAW filter, three interdigital (hereinafter referred to as "ID") electrodes are arranged in proximity with each other at an equal pitch on a 36.degree.-Y-cut X-direction-propagating LiTaO.sub.3 piezoelectric substrate, and reflectors are disposed at both sides of a region where the ID electrodes are located.
However, in the foregoing conventional longitudinally-coupled-type SAW resonator filter, unwanted spurious responses are generated in a frequency range higher than the pass band. FIG. 3 illustrates the attenuation-vs.-frequency characteristics of a known longitudinally-coupled-type SAW resonator filter. The solid line A indicates an enlarged essential portion of the characteristics represented by the solid line B with a scale on the right side of the vertical axis of FIG. 3. The longitudinally-coupled-type SAW resonator filter having the characteristics shown in FIG. 3 has a pass band from 864 to 868 MHz. In the above-described characteristics, large spurious responses indicated by the arrow C appear in a frequency range higher than the pass band, i.e., in the vicinity of 890 MHz, where a sufficiently large attenuation cannot be obtained.
A method for suppressing the above-described spurious responses C is disclosed in Japanese Unexamined Patent Publication No. 8-191229. This method includes a technique for differentiating a distance between a center ID electrode and one peripheral ID electrode from a distance between the center ID electrode and the other peripheral ID electrode. The longitudinally-coupled-type SAW resonator filter disclosed in this publication is shown in FIG. 4.
A longitudinally-coupled-type SAW resonator filter generally indicated by 10 has a structure in which a first ID electrode 14 and second and third ID electrodes 12 and 13 are arranged side by side on a piezoelectric substrate 11 in the direction in which a surface wave propagates. More specifically, the second and third ID electrodes 12 and 13 are respectively disposed at oppositeg sides of the first ID electrode 14. Reflectors 15, 15 are disposed at one side of the ID electrode 12 and at one side of the ID electrode 13, respectively, in the surface-wave propagating direction.
The second and third ID electrodes 12 and 13 are connected to an input terminal 21, while the first ID electrode 14 is connected to an output terminal 22. A voltage is applied to the ID electrodes 12 and 13 to generate a surface wave. The excited surface wave is then trapped in a range between the reflectors 15, 15 to generate a standing wave, and an output in accordance with the standing wave is extracted from the first ID electrode 14.
In the SAW resonator filter 10, the distance between the first ID electrode 14 and the second ID electrode 12, i.e., the center distance L.sub.1 between the most adjacent electrode fingers of the respective ID electrodes 14 and 12, is differentiated from the distance between the first ID electrode 14 and the third ID electrode 13, i.e., the center distance L.sub.2 between the most adjacent electrode fingers of the respective ID electrodes 14 and 13.
However, when the SAW resonator filters 10 illustrated in FIG. 4 are connected in two stages, the following problem occurs. In this filter 10, the center distances L.sub.1 and L.sub.2 are differentiated, which thereby generates an attenuation pole designated by the arrow F in FIG. 5 in a range higher than the pass band where a large attenuation can be obtained. On the other hand, the foregoing large unwanted spurious responses C can only be slightly suppressed and are not sufficiently prevented, as indicated by the attenuation-vs.-frequency characteristics illustrated in FIG. 5. In FIG. 5, the solid line D represents an enlarged essential portion of the characteristics indicated by the solid line E with a scale on the right side of the vertical axis of FIG. 5. Upon comparison of FIG. 5 with FIG. 3, it is seen that the unwanted spurious responses C are not significantly reduced.